Revision: Force and Types of Force General Science SSC (English Medium) 6th Standard Maharashtra State Board

1 dyne is that force which when acting on a body of mass 1 gram, produces an acceleration of 1 cm s^-2 in it.
1 dyne = 1 g × 1 cm s^-2.

The forces which are applied on a body through a connector, are called contact forces. Forces like Frictional force, Mechanical force, etc., are the forces of contact.

An inclined plane is usually a smooth, flat rigid surface inclined at an angle (θ) to the horizontal. It is used to raise heavy loads with a relatively small force. The longer the slope, the smaller is the effort needed.

Force is defined as the rate of change of linear momentum of a body with respect to time.

The gravitational force due to the earth on a body results in its acceleration. This is called acceleration due to gravity and is denoted by ‘g’. Acceleration is a vector.

The force experienced by a moving charge in the presence of a magnetic field, which depends on charge q, velocity v and magnetic field B, and which is opposite in direction on a negative charge compared to a positive charge, is called the magnetic force.

\[\overset{\rightarrow}{\operatorname*{F}}=\frac{d\overset{\rightarrow}{\operatorname*{p}}}{dt}=\frac{d(m\overset{\rightarrow}{\operatorname*{v}})}{dt}\]

or

\[\begin{array}
{rcl}\vec{F} & = & m\vec{a}
\end{array}\](if mass m is constant)

\[\vec{E}=\frac{1}{4\pi\varepsilon_0}\frac{Q}{r^2}\hat{r}\]

Magnetic force when v ∥ B: F = 0

Magnetic force when velocity is zero: ∣F_m∣ = 0

F = qv B sin θ

Maximum magnetic force (when v ⊥ B): F_max = qv B

If we stretch the index finger, middle finger and thumb of the left hand mutually perpendicular to each other such that the index finger points along the direction of the magnetic field and the middle finger along the direction of current (moving charge), then the thumb represents the direction of the force F experienced by the moving charge.

• Force is a Vector Quantity
• Unit of Force is Newton (symbol N) or kilogram-force (symbol kgf), where 1 kgf = g N if g is the acceleration due to gravity at that place (= 9·8 m s^-2 average value on the earth's surface). 

• Gravitational force — Force of attraction between any two masses; infinite range; weakest force (strength 10⁻³⁹); acts on all objects in the universe.
• Electromagnetic force — Attractive or repulsive force between charged particles; infinite range; strength 10⁻²; acts only among charged particles.
• Strong nuclear force — Binds protons and neutrons together in the nucleus; short range (10⁻¹⁵ m); strongest of all four forces (strength = 1).
• Weak nuclear force — Responsible for radioactive decay of atoms; very short sub-nuclear range (10⁻¹⁶ m); strength 10⁻¹³; acts among electrons, neutrinos, and subatomic particles.